Lens apertures are denoted as f/numbers, often referred to f/stops or ‘stops’. In the sequence f/2,is determined by dividing lens focal length by the diameter of the lens iris. So a 100mm focal length lens with an iris of 50mm will be f/2 (100 / 50 = 2) and 25mm will be f/4 (100 / 25 = 4). Modern lenses have f/stops controlled electronically from the camera body with one-half or one-third f/stop values often available. Physical aperture rings can still be found on some lenses.
